The present invention relates generally to exhaust sensors. More particularly, the present invention relates to an improved exhaust sensor and a method of preventing movement of a planar sensor element packaged in a mat.
Exhaust Sensors (or exhaust constituent sensors) have been used for many years in automotive vehicles to sense the presence of constituents in exhaust gasses (e.g., oxygen, hydrocarbons, nitrous oxides) and to sense, for example, when an exhaust gas content switches from rich to lean or lean to rich. One known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner.
Because automotive exhaust sensors are mounted to members of the vehicle exhaust flow system, the sensors must be durable, be able to withstand vibration and jarring such as would occur during installation and normal vehicle operation, and be able to withstand shock from the occasional stone or other small road debris that may happen to be thrown at the sensor, for example, by the vehicle""s tires.
Typically, great care is required when packaging and holding the flat plate sensing element within the outer housing (body) of the exhaust sensor. The flat plate sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, for example, glass and ceramics, typically have a high modulus of elasticity and cannot withstand much bending.
Sensing elements typically have the lower portion of the planar sensor protruding unsupported into the gas stream in a cantilever-type fashion. It is very important to support the portion of the element incident to the element""s lower portion properly, else the element will go into resonance and break. However, the element support cannot be too rigid, else it will transmit harmful vibrations to the element. This is a problem with conventional sensors that use ceramic or other rigid supports.
For example, glass seals are commonly used to bond to the element and prevent the element from moving. With glass seals, the planar sensing element is encased and held in proper position within a glass tube, which is itself bonded to a metal shield of the exhaust sensor. Unfortunately, these seals are rigid members that transmit shock pulses to the element. These shock pulses can, and often do, break the element. Glass seals are difficult to install, and they require high temperature cures sustained for long times.
Conventional sensors also are made using ceramic cements as element supports. Ceramic supports are also difficult to install and also have substantially long curing times. Like glass, these supports have a high modulus of elasticity and can transmit shock pulses to the element.
Talc Packs, which are often used as element supports in conventional sensors, are not only rigid enough to transmit shock pulses, but they also require a backing structure into which the talc pack must be compacted to enable compression of the element. The requirement for the backing structure severely limits design choice flexibility.
Accordingly, there remains a need in the art for a durable and rugged exhaust sensor having improved element supports that can be easily manufactured and easily installed.
The problems and disadvantages of the prior art are overcome and alleviated by the exhaust sensor and method of producing the same, the sensor comprising an elongated planar sensing element having a first end for contacting exhaust gas, a second and opposite end for connecting with at least one electrical terminal, and a central portion extending therebetween; a tubular shield within which at least a portion of said planar sensing element extends; a high temperature mat disposed between said tubular shield and said planar sensing element and about said central portion of said elongated planar sensing element; a shell for mounting said tubular shield to a conduit through which said exhaust gas flows, the shell having a lower shoulder for receiving a lower portion of said tubular shield; and either or both of (1) a disk element support, having an aperture through which at least a portion of said planar sensing element extends, the support disposed between the lower shoulder of said shell and said tubular shield containing the high temperature mat and (2) a vermiculite filled metal mesh support, disposed concentrically around the planar sensing element within the inner wall of the tubular shield.